Summer temperatures on the Norwegian archipelago of Svalbard in the High Arctic are now higher than during any time over the last 1,800 years, including a period of higher temperatures in the northern hemisphere known as the Medieval Warm Period, according to a new study. In an analysis of algae buried in deep lake sediments, a team of scientists calculated that summer temperatures in Svalbard since 1987 have been 2 to 2.5 degrees Celsius (3.6 to 4.5 degrees F) warmer than during the Medieval Warm Period, which lasted from roughly 950 to 1250 AD.

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Scientists say this year's record declines in Arctic sea ice extent and volume are powerful evidence that the giant cap of ice at the top of the planet is on a trajectory to largely disappear in summer within a decade or two, with profound global consequences.

The Medieval Warm Period is often cited by climate change skeptics as proof that the planet has experienced periods of high temperatures in recent centuries unrelated to the burning of fossil fuels. "Our record indicates that recent summer temperatures on Svalbard are greater than even the warmest periods at that time," said William D'Andrea, a climate scientist at Columbia University. The algae, which make more unsaturated fats in colder periods and more saturated fats in warmer periods, reveal critical clues about past climates.